Conventionally, when forming an SiC substrate/SiO2 insulating film interface of a MOSFET using SiC, an SiO2 insulating film is formed by subjecting the surface of the SiC substrate to hydrogen termination and then oxidizing the surface or forming an insulating film by deposition. At this time, since the hydrogen termination of the SiC substrate surface is easily removed and oxygen enters the inside of the substrate, oxidization proceeds.
In a MOSFET using an SiC (4H—SiC) substrate of a 4H structure, the mobility of the SiC substrate/SiO2 insulating film interface becomes extremely low. Even if thoughtful devising is made for the insulating film and interface, only the mobility (less than 100 cm2/Vs) that is far apart from the characteristic (1000 cm2/Vs) that 4H—SiC originally has is obtained.
On the other hand, in a MOSFET using an SiC (6H—SiC) substrate of a 6H structure, the mobility of the SiC substrate/SiO2 insulating film interface becomes sufficiently high. The withstand voltage of the 6H structure is 3.0 MV/cm and is equivalent to or higher than the withstand voltage (2.8 MV/cm) of the 4H structure. However, the electron mobility in the substrate used as a bulk is low and the structure is difficult to be used as a vertical MOSFET.
Further, in a MOSFET using an SiC (3H—SiC) substrate of a 3C structure, the mobility of the SiC substrate/SiO2 insulating film interface becomes sufficiently high. However, the device has a strong tendency to become normally on and is difficult to be used. Additionally, the withstand voltage of the 3C structure is 1.5 MV/cm and is insufficient in comparison with the withstand voltage (2.8 MV/cm) of the 4H structure.